Free piston internal combustion engines include one or more pistons which are reciprocally disposed within corresponding combustion cylinders. Each piston is typically rigidly attached to a plunger rod which provides a desired work output. For example, the plunger rod may be used to provide electrical power output by inducing an electrical current, or fluid power output such as pneumatic or hydraulic power output.
Pistons used in free piston internal combustion engines typically include a piston head which is entirely constructed from a metallic material such as aluminum or steel. Metals such as aluminum and steel have a relatively high coefficient of thermal expansion. Thus, during operation of the free piston engine, the metallic piston head expands considerably in the radial direction toward the inside surface of the combustion cylinder. Each piston head used in the free piston engine is thus formed with an outside diameter which provides a considerable radial clearance with the inside surface of the combustion cylinder to accommodate the relatively large radial expansion during operation. To prevent blow-by of combustion products past the piston head during operation, the outside peripheral surface of the piston head is formed with one or more piston ring grooves which receive corresponding piston rings therein. The piston rings allow for radial thermal expansion and contraction of the piston head, while at the same time effectively preventing blow-by of combustion products past the piston head.
Although piston rings provide valuable functionality as indicated above, it would be desirable to eliminate the use of piston rings to reduce manufacturing and assembly costs.
Moreover, to prevent excessive wear between the piston rings and the inside surface of the combustion cylinder, it is necessary to lubricate the piston rings with a suitable lubricant. The lubrication system for lubricating the piston rings may require additional porting and/or other structure to effect proper lubrication, which in turn increases the size and complexity of the engine. Additionally, the lubricating oil may increase undesirable emissions from the engine.
Another problem with using conventional piston and cylinder arrangements including a metallic combustion cylinder and metallic piston head with piston rings is that suitable fluid cooling channels must be provided within the combustion cylinder to effect the proper cooling of the combustion cylinder and piston head. These cooling fluid channels again increase the size and complexity of the engine.
The present invention is directed to overcoming one or more of the problems as set forth above.